It's sort of like trying to write 5 totally different books at the same time. With so much to think about, you don't have enough time to actually focus on the seperate stories. To create a good blog, you need to update it regularly and actually focus on it. I, myself, used to own over 8 blogs, which was way to much. I began to write more for one blog, thus the others fell in disrepair.
Do the Math!
Say you had 5 blogs, but you were only able to update them once every three or so days, on average (many blogs follow this schedule). 5 blogs multiplied by one post every three days equals 15 days between each posting (per blog). That's not good.
When I finally made the swich to Wordpress software, I decided that I would pick the blog topics that I most enjoyed, and incorporate them into Insane Hare; which I did. I also chose that instead of having a actual photoblog, I would use Flickr instead, which is practically already a photoblog.
(In the case of blogs only, of course.)
moreless the merrier!